The known navigation aid systems have means for computing trajectories between waypoints defined in a plan of the flight which can, for example, be entered by the pilot. The trajectories, computed at the start of the flight and possibly re-updated during the flight, are a support for the manoeuvres of the aircraft, whether decided by the pilot or by an automatic piloting system. In the known state of the art, the computed trajectory is split between a lateral trajectory, typically characterized by waypoints defined by a latitude and a longitude, and a vertical profile applied to this lateral trajectory to take into account constraints, for example of relief or of fuel consumption management.
Among the navigation aid systems, flight management systems, called FMS, are known of which a functional architecture is schematically represented in FIG. 1. In accordance with the ARINC 702 standard, they notably handle the functions of:                Navigation LOCNAV, 170, to perform the optimal locating of the aircraft as a function of geolocalization means (GPS, GALILEO, VHF radio beacons, inertial units, etc.),        Flight plan FPLN, 110, for inputting the geographic elements that make up the skeleton of the route to be followed (departure and arrival procedures, waypoints, etc.),        Navigation database NAVDB, 130, for constructing geographic routes and procedures from data included in the bases (points, beacons, intersection or altitude legs, etc.),        Performance database, PERF DB 150, notably containing the aerodynamic and engine parameters of the aircraft,        Lateral trajectory TRAJ, 120, for constructing a continuous trajectory from points of the flight plan, observing the aeroplane performance levels and the containment constraints,        Predictions PRED, 140, for constructing an optimized vertical profile on the lateral trajectory,        Guidance, GUIDANCE, 200, for guiding, in the lateral and vertical planes, the aircraft on its 3D trajectory, while optimizing the speed,        Digital data link DATALINK, 180, for communicating with the control centres, the infrastructures on the ground of the aircraft operators and of the other aircraft.        
From the flight plan FPLN defined by the pilot, a lateral trajectory is determined as a function of the geometry between the waypoints. From this lateral trajectory, a prediction function PRED defines an optimized vertical profile taking into account any constraints of altitude, of speed and of time. For this, the FMS system has performance tables PERFDB available, which define the modelling of the aerodynamics and of the engines. The prediction function PRED implements the equations of the aircraft dynamics. These equations are based numerically on values contained in the performance tables for computing drag, lift and thrust. By double integration, the speed vector and the position vector of the aircraft are deduced therefrom.
The taking into account of the meteorological conditions and changes thereof is added to the complexity of the computation of a flight trajectory. FIGS. 2a and 2b represent a great circle trajectory 10 between a point A and a point B, the x axis and the y axis corresponding respectively to the latitude and the longitude. The meteorological conditions in the environment of the trajectory are represented by means of a meshing M; the direction and the length of the arrows at each node of the meshing M illustrating the direction and the intensity of the wind vector at this node. Since the wind is not constant over the journey, the great circle trajectory 10, the geometrically shortest trajectory for linking A and B, does not prove to be the most economical in terms of fuel consumption and/or the fastest. A global trajectory optimization computation, such as dynamic programming for example, makes it possible to construct a trajectory 11 for linking the point A and the point B in a way that it is optimized, in terms of fuel consumption and/or time. Such a computation of a trajectory optimized as a function of the meteorological conditions requires significant computation resources and a lengthy computation time. This computation can be done in a computation station on the ground, but it is relatively unsuited to use in an embedded flight management system.
Enriching the trajectory computation of the embedded flight management systems of FMS type has been envisaged, by proposing means for diverting an aircraft from its trajectory on the basis of wind information. Thus, from the applicant, the patent document published under the reference FR2939505 is known, describing an embedded lateral trajectory optimization solution that relies on a local modification of the flight plan. The diversion is placed on the DIRTO function known to those skilled in the art, and described in the ARINC 702 standard. The trajectory is modified relative to the initial trajectory by adding a diversion point replacing a series of waypoints in the flight plan. The use of the DIRTO function necessarily restricts the complexity of the representation of the lateral trajectory to be followed. This implementation does not guarantee obtaining an optimal trajectory in terms of fuel consumption and/or time.
It therefore remains desirable to have effective navigation aid means available for adapting, on board the aircraft, a flight trajectory by making it possible to take account of a change in the meteorological conditions in order to optimize the cost of a path.